DIFFERENCE BETWEEN LOWEST POINTS AND ...points (LP) on each femoral condyle relative to the...

DIFFERENCE BETWEEN LOWEST POINTS AND CONTACT POINTS AS A REFERENCEIN 3D KINEMATIC ANALYSIS OF MEDIAL PIVOT TOTAL KNEE ARTHROPLASY

+*Iwakiri, K ; *Kobayashi, A ; *Minoda, Y; *Iwaki, H; **Kadoya, Y ; *Ohashi H; *Takaoka, K+* Osaka City University Graduate School of Medicine, Osaka, JAPAN

kentucky@msic.med.osaka-cu.ac.jp

INTRODUCTION:Although in-vivo 3D knee kinematics have been evaluated using

fluoroscopic image-matching techniques in many previous studies, nostandardized reference point for determining the kinematics of joints wasestablished; (e.g. the lowest point on femoral condyle relative to thetransverse axis of the tibial baseplate, contact position or the contactpoint determined by the centroid of the surface intersection) [1.2.3]. Thedifference of the lowest point and the contact point in posteriorstabilized and posterior cruciate retaining TKA has been reported [4].We have developed a Medial Pivot (MP) TKA with alumina femoralcomponent, and clinically used since 2001.

The aim of the present study was to evaluate the difference inkinematics between measurements using the lowest point and the contactpoint as a reference point in high conformity MP TKA for image-matching method [5].MATERIALS AND METHODS:

Knee kinematics was assessed in eight leftknees of eight patients after Medial Pivot TKA(Physio-knee; Japan Medical Materials, Japan, Fig.1). The mean age at operation was 74.7 years(range, 63 to 83), and the diagnosis in all patientswas osteoarthritis. All patients were female andwere followed clinically over three years. Themean flexion angle was 116 degrees (range, 100to 125). All patients signed an informed-consentform, and our Institutional Review Boardapproved the study.

To improve the accuracy in the image-matching method, fourφ1.0mm dimples (two balls in the anterior flange and one ball in eachposterior condyle) were made in the manufacturer at the accurate pointof the alumina femoral component in CAD data, and then φ0.8mmtantalum balls were inserted in those dimples during the cementing. Thetantalum balls were visible due to the radiolucency of the alumina. Forthe tibial component, the tips of three spikes were shaped to beaccurately detectable in the fluoroscope.

The patients were asked to examine the ipsilateral long-legbiplanar CR images (AP and 60° oblique) and to perform ipsilateralknee bending while being imaged at two frames per second with a flatpanel detector (FPD; Hitachi Medical, Clavis, Japan). A modelsilhouette was matched automatically with the 3D CAD silhouette bytranslating every 0.01mm and rotating every 0.01° of the 3D implantCAD model.

The biplanar CR images and a 3D CAD model of thecorresponding TKA were used to reproduce the spatial positions of thefemoral and tibial components in full extension without out-of-planeerror (Fig. 2). The knee images were examined at approximately every15° from full extension to maximum flexion by FPD. Based on the full-extension position of each component, the TKA components wereadjusted in all 6 degrees of freedom (6DOF) on calibrated FPD imagesto reproduce the in-vivo spatial position [5].

We evaluated tibiofemoral, anteroposterior (AP) and mediolateral(ML) translation by measuring 2 different types of points; (a) the lowestpoints (LP) on each femoral condyle relative to the transverse axis of thetibial baseplate, and (b) the contact points (CP) which was the nearestpoint between the surface of the femoral component and the surface ofthe polyethylene insert. Anterior and medial translation was positive.The out-of-plane error for this image-matching process was less than1.0mm. The paired t-test was used for analysis of points whencomparing LP with CP. Findings of p<0.05 were considered statisticallysignificant.RESULTS:

Both the LPs and CPs were observed to move in both the AP andML directions on the tibial plateau, and the difference was showed bymeans of illustration of each point (Fig. 3). In the AP direction of themedial condyle, LPs and CPs had significantly different points at 105°and maximum flexion, and the mean total excursion of the LPs was lessthan 2.1mm while that of the CPs was 6.0mm. In the ML direction, the

mean total excursion of the LPs was about 1mm in both condyles whilethat of the CPs was 8.5mm in the medial condyle and 6.0mm in thelateral condyle.DISCUSSION:

When comparing the two measurements methods, significantdifferences were observed in the medial condyle (Fig. 3). MP TKA hasball-in-socket shape in the medial femorotibial joint, which could causewider AP and ML translation of CPs than that of LPs (Fig. 4).

Schmidt et al. reported that metal medial pivot knee prosthesisrevealed medial pivot motion using contact position [6]. In the currentstudy, we examined the knee kinematics in deep knee bending of MPTKA, and found that they clearly exhibited medial pivot pattern from 0°to maximum flexion in LPs, while multi-directional translation wasobserved in CPs. The multi-directional translation suggested the force invaluable direction in this TKA, and then the ball-in-socket design inmedial femorotibial joint can successfully control in multi directionmovement.

The current study showed that the results of kinematic analysisusing LPs and CPs were different in high conformity implants. We haveto be aware of the difference and characteristics of both the referencepoints. For the motion analysis between femur and tibial component, theLPs might be useful. For the analysis of surface conditions such asfatigue and wear of polyethylene, the CPs might be available.

REFERENCES: [1] BanksSA, et al. CORR. 2004; 426:187-193. [2]Dennis SA, et al. CORR. 2004; 428:180-189. [3] Li G, et al. JBJS Am.2006;88: 395-402. [4] Pal S, et al. 51st ORS, poster No0571. [5] Sato T,et al. J Arthroplasty: 2004; 19: 620-628. [6] Schmidt R, et al. CORR.2003; 410:139-47.AFFILIATED INSTITUTIONS FOR CO-AUTHORS** Osaka Rosai Hospital, Sakai, Osaka, JAPAN

Fig. 3: Average AP and ML translation from 0° to maximum flexionin the measurements of A) lowest points and B) contact points.

Fig. 1: Physio-knee

Fig. 2: The spatial position of TKA components in full-extensionwas reproduced using image matching technique without out-of-plane error. (a) long-leg biplanar CR images (AP and 60° oblique).(b) 3D CAD models reproduced in-vivo joint position

(a) (b)

(A) (B)

Fig. 4: In the medial femorotibial joint inMP TKA, the contact point translation waswider than the lowest point translation insagittal plane.

53rd Annual Meeting of the Orthopaedic Research Society

Poster No: 1826